The Effect of P and Si Segregation to Grain Boundary on the Intergranular Corrosion of Type 304L Stainless Steel

The correlation between grain boundary segregation and intergranular corrosion in solution heat treated Type 304L stainless steel was examined. Grain boundary composition was analyzed with a high resolution analytical electron microscope (HRAEM). Intergranular corrosion susceptibility was tested in 9 kmol/m3 nitric acid solution containing 1 mol/m3 Ru at 373 K for 1.8 Ms. The followings are found: (1) The amount of phosphorus segregation to grain boundaries in solution heat treated stainless steels were successfully measured quantitatively with HRAEM. The amount of phosphorus segregation increases with higher phosphorus content, and the phosphorus concentration of grain boundary becomes about a hundred times of that of grain interior. (2) Phosphorus and silicon additions cause intergranular corrosion. The correlation between amount of phosphorus segregation and intergranular penetration depth is seen in low silicon content steels. But high silicon content steels show deep intergranular penetration depth, in spite of the small amount of phosphorus segregation. (3) Assuming MCLEAN'S model of equilibrium segregation, the segregation free energy of phosphorus at 1323 K is estimated as about 57 kJ/mol.